Modified track-etched membranes using photocatalytic semiconductors for advanced oxidation water treatment processes

Rossouw, Arnoux (2013-03)

Thesis (MScEng)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: The purpose of this study was to develop modi ed tract-etched membranes using nanocomposite TiO2 for advanced water treatment processes. Photocatalytic oxidation and reduction reactions take place on TiO2 surfaces under UV light irradiation, therefore sunlight and even normal indoor lighting could be utilised to achieve this effect. In membrane ltration, caking is a major problem, by enhancing the anti-fouling properties of photocatalysts to mineralise organic compounds the membrane life and e ciency can be improved upon. In this study the rst approach in nanocomposite membrane development was to directly modify the surface of polyethylenetherephthalate (PET) track-etched membranes (TMs) with titanium dioxide (TiO2) using inverted cylindrical magnetron sputtering (ICMS) for TiO2 thin lm deposition. The second approach was rst to thermally evaporate silver (Ag) over the entire TM surface, followed by sputtering TiO2 over the silver-coated TM. As a result a noble metal-titania nanocomposite thin lm layer is produced on top of the TM surface with both self-cleaning and superhydrophilic properties. Reactive inverted cylindrical magnetron sputtering is a physical vapour deposition method, where material is separated from a target using high energy ions and then re-assimilated on a substrate to grow thin lms. Argon gas is introduced simultaneously into the deposition chamber along with O2 (the reactive gas) to form TiO2. The photocatalytic activity and other lm properties, such as crystallinity can be in uenced by changing the sputtering power, chamber pressure, target-to-substrate distance, substrate temperature, sputtering gas composition and ow rate. These characteristics make sputtering the perfect tool for the preparation of di erent kinds of TiO2 lms and nanostructures for photocatalysis. In this work, the utilisation of ICMS to prepare photocatalytic TiO2 thin lms deposited on track-etched membranes was studied in detail with emphasis on bandgap reduction and TM surface regeneration. Nanostructured TiO2 photocatalysts were prepared through template directed deposition on track-etched membrane substrates by exploiting the good qualities of ICMS. The TiO2-TM as well as Ag-TiO2-TM thin lms were thoroughly characterised. ICMS prepared TiO2 lms were shown to exhibit good photocatalytic activities. However, the nanocomposite Ag-TiO2 thin lms were identi ed to be a much better choice than TiO2 thin lms on their own. Finally a clear enhancement in the photocatalytic activity was achieved by forming the Ag-TiO2 nanocomposite TMs. This was evident from the band-gap improvement from 3.05 eV of the TiO2 thin lms to the 2.76 eV of the Ag-TiO2 thin lms as well as the superior surface regenerative properties of the Ag-TiO2-TMs.

AFRIKAANSE OPSOMMING: Die doel van hierdie studie was om verbeterde baan-ge etste membrane (BMe) met behulp van nano-saamgestelde titaandioksied (TiO2) vir gevorderde water behandeling prosesse te ontwikkel. Fotokatalitiese oksidasie- en reduksie reaksies vind plaas op die TiO2 oppervlaktes onder UV-lig bestraling, en dus kan sonlig en selfs gewone binnenshuise beligting gebruik word om die gewenste uitwerking te verkry. In membraan ltrasie is die aanpaksel van onsuiwerhede 'n groot probleem, maar die verbetering van die self-reinigende eienskappe van fotokatalisators deur organiese verbindings te mineraliseer, kan die membraan se leeftyd en doeltre endheid verbeter word. In hierdie studie was die eerste benadering om nano-saamgestelde membraan ontwikkeling direk te verander deur die oppervlak van polyethylenetherephthalate (PET) BMe met 'n dun lagie TiO2 te bedek, met behulp van reaktiewe omgekeerde silindriese magnetron verstuiwing (OSMV).Die tweede benadering was eers om silwer (Ag) termies te verdamp oor die hele BM oppervlak, gevolg deur TiO2 verstuiwing bo-oor die silwer bedekte BM. As gevolg hiervan is 'n edelmetaal-titanium nano-saamgestelde dun lm laag gevorm bo-op die oppervlak van die BM, met beide self-reinigende en verhoogde hidro liese eienskappe. OSMV is 'n siese damp neerslag metode, waar materiaal van 'n teiken, met behulp van ho e-energie-ione, geskei word, en dan weer opgeneem word op 'n substraat om dun lms te vorm. Argon gas word gelyktydig in die neerslag kamer, saam met O2 (die reaktiewe gas), vrygestel om TiO2 te vorm. Die fotokatalitiese aktiwiteit en ander lm eienskappe, soos kristalliniteit, kan be nvloed word deur die verandering van byvoorbeeld die verstuiwingskrag, die druk in die reaksiekamer, teiken-tot-substraat afstand, substraattemperatuur, verstuiwing gassamestelling en vloeitempo. Hierdie eienskappe maak verstuiwing die ideale hulpmiddel vir die voorbereiding van die verskillende soorte TiO2 lms en nanostrukture vir fotokatalisasie. In hierdie tesis word OSMV gebruik ter voorbereiding van fotokatalitiese TiO2 dun lms, wat gedeponeer is op BMe. Hierdie lms word dan in diepte bestudeer, met die klem op bandgaping vermindering en BM oppervlak hergenerasie. Nanogestruktureerde TiO2 fotokataliste is voorberei deur middel van sjabloongerigte neerslag op BM substrate deur die ontginning van die goeie eienskappe van OSMV. Die TiO2-BM dun lms, sowel as Ag-TiO2-BM dun lms, is deeglik gekarakteriseer. OSMV voorbereide TiO2 dun lms toon goeie fotokatalitiese aktiwiteite. Nano-saamgestelde Ag-TiO2 dun lms is egter ge denti seer as 'n veel beter keuse as TiO2 dun lms. Ten slotte is 'n duidelike verbetering in die fotokatalitiese aktiwiteit bereik deur die vorming van die Ag-TiO2 nano-saamgestelde BMe. Dit was duidelik uit die bandgapingverbetering van 3,05 eV van TiO2 dun lms in vergelyking met die 2,76 eV van Ag-TiO2 dun lms. 'n Duidelike verbetering is behaal in die fotokatalitiese aktiwiteit deur die vorming van die Ag-TiO2 nano-saamgestelde TMs.

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